In semiconductor technology, photoresists situated on a substrate are more and more often being patterned by means of electrically charged particles, for example electrons. In this case, patterning by means of electron beam lithography enables more highly resolved structures than optical lithography techniques. Principally in the fabrication of lithography masks, which generally comprise a quartz glass substrate with an applied patterned metal layer, for example chromium, the structures to be produced in the metal layer and the quartz glass substrate are becoming ever smaller. In this case, the lithography masks have structures that are transferred by means of exposure, e.g., using UV radiation, to a photoresist situated on a semiconductor substrate to be patterned. The fine structures of the lithography masks can be attributed to the fact that so-called OPC structures (optical proximity correction) are often integrated into the masks, the OPC structures being significantly smaller than the structures that are actually to be imaged. The OPC structures comprise, e.g., very small structures that serve to compensate for the modification of the mask structure by the light.
Furthermore, the fabrication of so-called phase shifter masks is particularly demanding since additional layers have to be applied to the mask in this case, or the mask substrate is removed in a defined manner after the patterning of the metal layer in order to achieve, by means of interference, the desired phase jumps during the exposure of a photoresist on a semiconductor substrate.
The fabrication of the phase shifter masks generally requires two separate lithographic patterning steps of photoresists. In the first step, a metal layer, for example chromium, situated on the mask substrate, the quartz glass substrate, is patterned. For this purpose, a photoresist layer is applied to the chromium layer and is patterned by means of an electron beam and subsequently developed. Since a continuous metal layer is present on the mask substrate, the negative electrical charging of the mask that arises can still be dissipated without any problems. The structure present in the photoresist can then be transferred to the chromium layer by means of an oxygen/chlorine plasma, for example. In the fabrication of phase shifter masks, it is subsequently often necessary, moreover, for the quartz glass substrate likewise to be patterned (see, e.g., FIG. 1G). For this purpose, a photoresist is likewise applied to the mask with the prepatterned chromium layer and is subsequently patterned by means of an electron beam. On account of the already prepatterned chromium layer that is no longer present on the entire quartz glass substrate, whole-area dissipation of the charge is no longer possible here, with the result that the quartz glass substrate is negatively charged during the writing operation. This negative charging influences the electron beam that is necessary both for writing and for alignment control. This influencing leads to an undesirable deflection and expansion of the electron beam, which is particularly disturbing during alignment, but also leads to undesirable distortions and errors during the patterning of the photoresist. For this reason, in the second lithography step, the photoresist has often been patterned optically heretofore, but this has the disadvantage that the resolutions are not as high as those that can be achieved in the case of patterning by means of an electron beam.